Volume expander with timing control



Dec. 25, 1951 J. H. HAMMOND, JR

VOLUME EXPANDER WITH TIMING CONTROL Filed Jan. 6, 1948 .OII'III lll|lil AAA Mr 75 7'0 74- NV I 76 79 "Ia-H I (P i so a! 82 c 11 7 5'51 s RHYTHM SERVO 858685 87 as 9 J ANALYZER o MECHANISM 6 v 84 F i I l INVENTOR JOHN HAYS HAMM:ND,JR.

Dec. 25, 1951 J. H. HAMMOND, JR

VOLUME EXPANDER WITH TIMING CONTROL Filed Jan. 6, 1948 2 SHEETS-SHEET 2 0 H2 uz H7 H4- l I m H6 8 I I H lll)l0ll.l -|a: 1

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Patented Dec. 25, 1951 UNITED. STATES PATENT OFFICE VOLUME EXPANDER WITH TIMING CONTROL John Hays Hammond, J r., Gloucester, Mass.

Application January 6, 1948, Serial No. 755

This invention relates to volume expander circuits for record players and more particularly to a volume expander circuit having an automatic control for the timing of the build up and decay of the amplifier gain.

An object of the invention is to provide a system which automatically varies the timing of the change of the amplifier gain in accordance with the rhythm of the signal.

For the average symphonic reproduction the time constant of build up and decay of the amplifier gain should be relatively long while for speech and dance records the time constant should be relatively short. This timing may be manually controlled by changing the capacitance of the filterin and timing condensers or by changing the resistance value of the filtering circuit. However, change f resistance values disturbs the final value of the voltage that will rep" resent the steady state condition, whereas capacitance changes do not disturb the voltage.

In order to change the. timing by varying the capacitance it is necessary to use a capacitance range of the order of .05 microfarad to 2.0 microfarads in the expander timing circuit. It i not commercially practicable to produce such large changes in capacitance by electronic means. It is accordingly another object of the invention to provide a suitable control means responsive to the rhythm frequency for producing the necessary capacitance changes in the timing circuit.

Various other objects and advantages Will be apparent as the nature of the invention is more fully disclosed.

In accordance with the present invention the signal is passed through a rhythm analyzer circuit including a low pass filter having an upper cut-off at a frequency of the order of cycles per second. The analyzer is adapted to produce a direct voltage which is directly'proportional to the rhythm frequency. This voltage is applied through a servo mechanism or a mechanical relay to control a switch which is adapted to connect individual condensers of a condenser bank successively in the expander timing circuit. The arrangement is such that an increase in rhythm frequency causes a decrease in capacity of the expander timing circuit which adapts the expander to respond more rapidly to the changes in volume level. On the other hand, a decrease in the rhythm frequency results in the intro-duction of more capacity into the timing circuit so as to cause the amplifier to respond less rapidly to changes in the volume level. The control is so selected that the amplifier is made to respond most effectively to the. particular type of signal being reproduced.

Although the novel features which are characteristic of this invention are pointed out more particularly in the claims, the nature of the invention will be better understood'by referring to the following with the accompanying drawings in which a description taken in connection- 7 Claims. (Cl. 178 14) specific embodiment thereof has been set forth for purposes, of illustration.

'In the drawings:

Fig. 1 is a schematic-diagram of an expander circuit embodying the present invention;

I Fig. 2 is a schematic diagram of one type of rhythm analyzer to be used in the circuit of Fig. l; and

I Fig. 3 is a schematic diagram of one type of servocontrol mechanism to be used in the circuit of Fig. 1.

Referring to the system shown in Fig. 1, the expander or dynamic multiplier stage is shown as comprising a pair of pentodes Ill having signal grids II which are driven in push-pull relationship from the output of a signal inverter tube A 2. The signal inverter'circuit is of the usual construction and includes signal input lines l3 and it which receive the signal from the pick-up device such as a phonograph pick-up of any desired type and apply the signal voltage across a resistor IE to the input circuit of the tube 12 which includes a signal grid It and a cathode I! which is connected through cathode resistors l8 and id to the ground lea-d M. The tube I2 is pro vided with an anode 2B which is supplied through a resistor 2|, from a potential source shown as a battery 22 and is connected through a blocking condenser 23 to the signal grid ll of the first pentode [6. The junction of the resistors l8 and i9 is connected through a blocking condenser 2d and a lead 25 to the signal grid H of the second pentode it. A condenser 26 by-passes the signals around the resistor 18. The arrangement is such that equal and opposite signals are applied to the signal grids ll of the two pentodes for causing the latter to operate as a push-pull amplifier.

The anodes 30 of the pentodes ID are connected to the outer terminals of the primary 3| of a push pull transformer 32 having a secondary 33 which is connected to a power amplifier not shown. .The center tap of primary 3| is connected to the positive end of battery 22, and also through ballast resistors 36 to the anodes of the pentodes. The center tap is also connected to the second or screen grids 3'! of the pentodesthrough screen resistor 35, and to the cathodes 33 by resistor 36.. Suitablebias for the first grids 1 to which the signal is applied and for the third grids to which the expansion control voltage is applied is provided by resistors and capacitors connected from the cathodes 38 to ground line 14.

An amplifier tube 40 is provided with a signal grid El which is driven by the voltage across the resistor 19, which in turn constitutes the voltage applied to the signal grid l I of the second pentode iii. For this purpose the signal grid 4! is connected by a lead t2 through a blocking condenser 43 to the junction of the resistors I S and IS. The output of the amplifier tube 4c is fed to a potentiomete'r t4 having a variable tap d5 which is con-' nected bya lead 45 to the control grid 41 of a rectifier driver tube 48. The anode A9 of the tube 40 is supplied with D. C. voltage through a resistor 58 and a lead from source shown as a battery '52, and is connected through a blocking condenser 53 and a lead 54 to the potentiometer 44.

The rectifier driver tube 48 is provided with an output circuit including resistors 58 and 59 and a pair of rectifiers 80 and GI connected in push-pull relationship to produce a, unidirectional voltage across a resistor 62.

The anode 63 of the tube 48 is supplied from the battery 52 through a resistor 54, and the cathode is connected to ground lead l4 through resistors 65 and 66 in series, one of which may be bypassed by capacitor 61. The cathode and anode of the tube 48 are connected to drive the rectifier system 58 to 62 by coupling condensers 68 and 68. Constants are suitably chosen so that the driving voltages for the two rectifiers are substantially equal butopposite in phase, and represent a suitable acoustical evaluation of the electrical signal at input 13 and Hi. The output circuit of the rectifiers 60 and 6| is connected to a filter circuit comprising resistors 62, 18, H and capacitors l2, [3 to produce a unidirectional voltage across resistor H which is supplied by means of a lead 14 through a resistor to the third grids of the pentodes ill, It so as to control the amplification of said pentodes in accordance with the signal level in a manner well known in expander circuits. The constants of the filter circuit including capacitors l2 and 13 are chosen for the most rapid rate of crescendo and decrescendo that may be desired when a signal is impressed and withdrawn from the input terminals. This may be a very rapid rate in view of the push pull type of rectification used.

For providing slower rates of operation, a connection block is provided with terminal 16 connected by line H to the ground end of condenser 13, and terminal 18 connected by line 14 to the other end which feeds the third grids of the pentodes with control voltage. The terminal I8 is connected to one end of capacitor 19, the capacitance of which when thrown in parallel with that of 13 determines the slowest rate of speed of expander action. The other end of condenser 19 is connected to a set of capacitors 88, 8!, 82 in series. A sliding switch is provided with the movable contact 83 connected by line 84 to the grounded terminal 15, and with fixed elongated contacts 85, 85, 81, 88 and 89 across 85 to 88 of which capacitors 80 to 82 are connected, with no capacitor between contacts 88 and 89. This switch is of the shorting type so that at least one switch contact is always connected through the movable contact to ground.

When the movable contact 83 is on contact 87 as shown, the added capacitance across connection block i5, 18 is that due to capacitors 19, 8D and 85 in series. If it is moved to the right, the added capacitance is diminished and becomes zero when on contact 89. Or if it is moved to the left, the added capacitance increases and becomes that of capacitor 19 when on contact 85. Because of the shorting features, the capacitance is made to vary in steps without the added capacitance falling to zero. The response time of the dynamics of the expander therefore is made to be controlled by the motion of the switch contact.

The contact 83 is shown as actuated by an output link of a servo mechanism 9| which is adapted to shift the contact successively across the contacts 85 to 89. The servo mechanism 9! is connected to be driven by the output of a rhythm analyzer 92 the input of which is connected by a lead 93 through a blocking condenser 94 to the anode 49 of the amplifier tube 40. The rhythm analyzer 92 and the servo mechanism 9i are adapted to drive the contact 83 in a direction to increase the total capacity in response to a decrease in the rhythm frequency and to decrease the total capacity. in response to an increase in rhythm frequency. In this way the timing of the control voltage applied to the control grids of the pentodes l0 varies in accordance with the characteristics of the signal so as to produce the result above specified.

The rhythm analzyer 92 may have various characteristics and may be of any desired type; One embodiment is illustrated in Fig. 2 which shows an amplifier tube I08 driven by the signal received through the lead 93 from the amplifier tube 40 of Fig. 1. The amplifier tube IE0 is provided with an output circuit including an anode llll which is connected to a rectifier 02, operating into a low pass filter I03. The filter I85 may have an upper cut-off of for example 10 cycles and it may be of the R-C or L-R-C con struction. In the embodiment shown the amplifier tube Hill is provided with the usual bias resistors and sources of voltage supply, and the rectifier I02 is connected to develop a unidirectional voltage across a resistor I04 which constitutes the input resistor to the filter circuit I03,

Inasmuch as the output of the filter I53 is olependent upon the strength of the signal as well as upon the frequency, the output is connected across a resistor I08 to a pair of amplifier limiter tubes I09 and H0 which are resistance-capacity coupled through a resistor iii and are adapted to produce suflicient amplification so that the magnitude of the output of the tube i 10 is inde-- pendent of the input signal level to the filter Hi3 and the wave form is a function only of the ire quency of the input signal. The amplifiers 5G9 and I ID are provided with the usual coupling and voltage supply circuits. The capacitances of the blocking condensers, the tubes and the plate and grid resistors are so chosen that the amplifier will operate satisfactorily down to below 1 cycle per second. A representative system has been shown merely for illustration. In certain instances a more complex type of amplifier may be required.

The fixed level output of the limiter tube H0 is fed through a tapered filter including capacitors H2, I i3 and resistor H4 to a rectifier H5 which develops a unidirectional voltage across a resistor H6. This tapered filter is arranged to efiiciently transmit rhythm frequencies of the highest value of say 10 cycles per second, and poorly transmit frequencies near or below 1 cycle per second. As the dynamic activity increases so that low subaudible frequency pulses are transmitted to the rectifier, the D. C. output increases roughly in proportion to the rhythm frequency and more or less independent of the signal volume. In case there is no dynamic activity and the sound level is being maintained fixed, there is no output voltage developed across resistor H6. It will be understood that for very Weak signals, the limiter tube H0 may not be saturated, and the input to condenser H2 may be resistor II6 is connected to output leads I" and H8 which in turn are connected to the input of the servo mechanism 9|.

The servo mechanism 9| may also be of any desired type. A representative type is shown in Fig. 3 as comprising a grid controlled rectifier I having an input circuit connected to the leads II I and H8 and having a plate supplied through a transformer I2I from an A. C. supply line I22. work into a solenoid I23 and is arranged so that the amount of rectified current depends upon the bias of the grid of the tube 20. The arrangement is such that for zero D. C. input from the rhythm analyzer 92 the rectified output current will be a minimum. This corresponds to the switch position for closing the contact 85. The solenoid I23 is connected to drive the link 90 and is retracted by a spring I24 so that with increased current through the solenoid I23 the link 90 and the movable contact 83 are shifted to the right whereas with a decreased current the spring I20 returns the contact 83 to the left.

In the operation of this system it will be noted that the input signal from the phonograph pickup or the like is supplied to the amplifier tube I2 which is adapted to produce a push-pull signal which is supplied to the signal grids of the amplifier pentodes I 0. The output of these pentodes is applied to the power amplifier and then to th speaker in the usual manner. The gain of the pentodes I0 is controlled by the voltage applied to the third grids thereof from the lead I4. This voltage is a function of the output of the push-pull rectifiers 60 and GI which in turn are driven by the rectifier .driver amplifier 48 and by the amplifier 40. The timing of the circuit including the filter network I0 to 13 is such that this voltage represents the average value of the signal level over a predetermined time period and the control period or the rate of change of the voltage across the resistor II is dependent upon the value of the capacitors connected across the condenser I3. Hence the rate of change of the control voltage in response to change of signal level is varied by the setting of the switch contact 83.

The signal which is supplied to the signal grid of one of the pentodes I0 is connected through the lead 42 to drive the amplifier 40 which in turn is connected through a condenser 94 and lead 93 to drive the rhythm analyzer 92.

The rhythm analyzer 92 in the form shown in Fig. 2 includes an amplifier I00 which further amplifies the signal and drives the rectifier I02 which in turn, through the low-pass filter I03, drives the amplifier tubes I09 and I I0 which serve to eliminate fluctuations due to variations in signal level and to produce an output voltage across the resistor I I6 which is a function of the rhythm frequency or the rate of change of signal level. This is applied through the leads II! and I I8 to the servo mechanism 9| which in the form shown in Fig. 3 includes a solenoid driven from a transformer I2I through a grid controlled rectifier I20 controlled in accordance with the output of the rhythm analyzer and adapted to shift the contact 84 as above specified.

It will be evident from the above that the system operates automatically to provide a timed expansion which is varied in accordance with the timing requirements of the signal being reproduced. The system is accordingly suited for the reproduction of a wide variety of, signals from symphonic to speech and dance music and in each The transformer I2I is connected to case produces the most desirable and pleasing characteristics.

Although a specific embodiment of the invention has been shown for purposes of illustration, it is to be understood that various changes and adaptations may be made therein and that the invention is only to be restricted in accordance with the scope of the following claims.

What is claimed is:

1. An expander system comprising an input circuit for supplying signals to be amplified and expanded in volume range, an amplifier stage connected to amplify said signals, a gain control circuit connected to said input circuit and having signal-responsive means, including a timing capacitance, responsive to the average volume level of the signal over a predetermined time period and connected to control the gain of said amplifier stage, variable timing means in said gain control circuit connected to control said time period, said variable timing means includin a rhythm analyzer circuit including a low-pass filter net work having an output voltage substantially directly proportional to frequency, said rhythm analyzer being connected to said input circuit and responsive to the rhythm frequency to develop a control voltage substantially directly proportional to said rhythm frequency, said timingmeans also including actuator elements responsive to said control voltage and. connected to alter the timing capacitance in accordance with the rhythm frequency.

2. An expander system as set forth in claim 1 in which timing capacitance comprises a fixed capacitor and said actuating circuit is connected to vary the effective capacity of said capacitor.

3. An expander system as set forth in claim 1 in which said timing capacitance comprises a condenser bank having a plurality of contacts connected to the individual condensers of the bank, a movable contact is connected to selectably engage one of said plurality of contacts, and said actuator elements are connected to move said movable contact.

4. An expander system according to claim 3 in which said actuator elements include a servo member having mechanical means connected to actuate said movable contact.

5. An expander system according to claim 4 in which said servo mechanism comprises a solenoid which is energized by said control voltage.

6. An expander system according to claim 1 in which said rhythm analyzer comprises a low pass filter having a high frequency cut-off at about 10 cycles per second and which'is connected to develop a control voltage proportional to the frequency passed by said filter.

7. An expander system as set forth in claim 6 in which a limiter amplifier stage is included with said low pass filter to substantially eliminate fluctuations in control voltage due to changes in rhythm intensity.

JOHN HAYS HAMMOND, JR.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,979,035 Hammond Oct. 30, 1934 2,221,541 Hathaway Nov. 12, 1940 2,404,160 Boucke July 16, 1946 2,410,178 Milasus Oct. 29, 1946 

